Guanin nucleotide binding proteins, G proteins, function as receptor effector couplers. Two distinct G proteins, Gs and Gi, couple stimulatory and inhibitory hormone receptors respectively to adenylate cyclase. A homologous retinal protein, transducin, couples a light receptor, rhodopsin, to cGMP phosphodiesterase. We have purified transducin, raised antibodies to it, and used these to compare the structure and function of Gs, Gi, and transducin. These studies provide important insights into the mechanism of signal transduction across the cell membrane.